The invention contrives to provide a reliable, fast, light weight, compact, low power consumption automatic focusing system for portable devices such as cellular phone camera.
As the position of an object changes, a focus distance representing a distance between a lens and a plane that a focused image of the object is formed also changes. To form the focused image of the object on the image sensor automatically, a sensor distance representing a distance between the lens and the image sensor has to be the same as the focus distance. The focus distance or the sensor distance can be defined in various ways depending on the optical arrangements of optical elements. In the conventional automatic focus systems, the sensor distance is matched with the focus distance by moving one or more optical elements such as lens, mirror and sensor. The majority of the conventional automatic focus imaging systems perform automatic focus by moving one or more lenses equipped with electro-magnetically driven motors and/or piezo-electrically actuated apparatus. Since the lens or lenses in those systems have a considerable inertia and need to have macroscopic mechanical motions, the automatic focus imaging systems require a macroscopic actuator generating large actuating force. The macroscopic actuator can pose many problems including low focusing speed and low portability due to the increase of volume, mass and power consumption. Alternatively, the automatic focus can be performed by using a movable sensor. But, it also requires a macroscopic actuator with additional complexity to satisfy electrical connection. For simpler automatic focus, a movable mirror can be used. While the movable mirror can provide a simple and reliable automatic focus, it still requires a macroscopic actuator such as voice coil.
To apply the automatic focus imaging system to portable devices such as cellular phone camera, it is very important to reduce the volume, mass and power consumption of the automatic focus imaging system and increase the reliability and speed of automatic focus function.